What is an alternator? How To Detect A Faulty Alternator?

You often read articles that begin with “The battery light turns on suddenly” or “The battery light turns on while driving”. But in reality, their technically correct designation is not the battery light, but the charge control light. 

This means that this light comes on if the car battery is not being charged by the generator (the alternator) and the onboard power is supplied only by the car battery. 

When the engine is not running but the ignition is switched on, it is normal for the charge lamp to be on. If the light does not go out after starting or comes on while driving, there is a presumption that there is a defect in the alternator.

What is the function of the alternator?

In the past, the alternator was mainly used to supply electricity to the vehicle’s lighting system. Today, the alternator has to provide power not only for the lighting system but also for the starter battery and various other electrical equipment, such as window lifters.

The basic principle of an alternator is always the same: part of the crankshaft rotation energy is converted into electrical energy. If the generator is not built directly around the crankshaft, the rotational motion of the crankshaft must first be transferred to the alternator. 

In most cases, this is done via a V-belt or V-belt. The most important elements of the alternator are the charge controller, roller bearings, carbon brushes, diodes, freewheel and slip rings. . The easiest way to check and replace the charge controller is to use a V-belt or V-belt.

How do you recognize a faulty alternator?

If the alternator is not producing enough voltage or is not producing voltage at all, the charge light will turn on and indicate that the alternator is not working properly. The most common cause is the wear of the carbon brushes on the charge controller. However, this does not mean that the entire alternator is faulty. Sometimes the V-belt simply tears or the effect of the belt tensioner wears out.

On vehicles that still have a bulb as a charge indicator light, the charge indicator light may flicker when the vehicle is idling. In this case, the charge voltage at low engine speed is too low, which may be due to the alternator itself, the carbon brushes, the regulator, the battery or an oxidized contact.

Mechanical defects on alternators are quite rare. In reality, only the roller bearings, freewheel or slip rings and carbon brushes of the alternator can wear out. With the exception of shoes and carbon brushes, these defects are usually heralded by screeching, whistling or creaking alternator bearings.

A coil closure in the stator or even in the rotor is very rare. If you notice, it is for the glow of the charge indicator light, but that also doesn’t have to be the case. In the worst case, you will only notice it when the engine stops starting.

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How can an alternator be checked?

Check the charging voltage

There are several things you can check on an alternator. First, you should always check the charging voltage during operation. A simple multimeter is enough, which is set to 20 V DC and whose measuring tips are held at the poles of the car battery. The charging voltage should be between 14 and 14.8V. 

In older vehicles, the charge controller setting limit may also be below 14V, but should not be below 13.5V. possibility of a cable defect, the charging voltage can also be checked directly on the alternator. 

To do this, if only the battery voltage itself or a slightly lower voltage is measured on the alternator, the alternator will not charge. In this case, check if there is the voltage at the excitation current connection. 

To do this, simply switch on the ignition, the engine must not run. In order not to get an incorrect result, the connection from the excitation line to the alternator must also be disconnected. The excitation current line is always the second thinnest line. 

To measure the excitation voltage, one pole must be connected to this line and the other pole to the housing or motor block. If an excitation voltage is present, the fault is in the alternator. If there is no supply voltage, the fault is in the supply line (eg.

If a charging voltage can be measured on the alternator, the fault lies in the supply line from the alternator to the battery (e.g. defective fuse in front of the battery). If nothing can be measured directly on the alternator, there is a fault in both cables or the battery cable and alternator (very unlikely).

Check the charge controller

If during operation the alternator does not have a voltage higher than the open-circuit voltage of the battery, in most cases the carbon brushes of the regulator are worn or the regulator itself is defective. 

In both cases, the cheapest option is to simply change the regulator. On some vehicles, it is not even necessary to remove the alternator. The regulator itself can only be checked with respect to its regulation limit. 

This means that you measure the voltage at any point during the operation and give the engine several bursts of gas. The control limit must not be exceeded. If you want to be sure that only the regulator is faulty, you need to remove the alternator and often disassemble it as well. 

Then you can check the blocking direction of the individual diodes. To do this, set the multimeter to measure continuity and hold the measuring tips on one side of each diode, that is, on one contact each. The measurement is then carried out again in reverse. 

The current must flow in one direction and not the other. If a diode has continuity in both directions or has none at all, it is defective.

Measure the voltage winding from the outside

Theoretically, the voltage winding could also be measured from the outside. To do this, only the regulator must be removed and the two pins held against the two measuring pads. The voltage winding must have a resistance of 3 to 5 Ohm. 

Additionally, the insulation resistance can be measured by holding one pin on one of the two roller shoes and the other pin on the alternator box. The largest possible measuring range for resistance should be set on the multimeter and the multimeter should show “OL” for “open load”. 

This means that the resistance is outside the measuring range and the excitation coil, therefore, has no earth connection to the housing. If the multimeter shows a resistance of 0 Ohm, the voltage coil has a short circuit to ground and is therefore defective. For safety reasons, the insulation resistance can also be measured against the hook rotor. 

To do this, it is best to hold the second pin against the screw or nut in the center of the pulley. It is also recommended that a visual inspection of the manifolds inside the alternator be made immediately after removing the regulator. A dark spot and edge deposit indicate bad contact.

Measure the stator windings

The stator windings cannot be measured individually because they are all connected to each other at one end. Therefore, it is possible to measure the resistance of two coils at the same time only by always testing two wires connecting the stator windings at the diodes with the measuring tips of the multimeter and comparing all the values with each other. 

The resistance measurement values should be approximately the same. Again, the insulation resistance of a stator winding connection wire must be measured with respect to the housing. If the multimeter shows “OL” in the highest resistance measurement range, everything is fine.

Check the freewheel and roller bearings

The mechanical side of the alternator can be easily checked in the installation conditions. First of all, the V-belt must be removed from the alternator pulley. The easiest way to do this is to loosen the V-belt at the tension pulley. In principle, two things can now occur the function of the freewheel and the axial and radial play of the roller bearings.

You can check the freewheel by locking the alternator fan wheel with a screwdriver while turning the pulley. If the freewheel works normally, the pulley can be turned in one direction and not the other. If the freewheel is defective, the pulley cannot be turned in any direction. 

To avoid a short circuit, it is always best to disconnect the car battery at this point. The axial play of the rolling bearings can be checked by trying to carefully pull and push the pulley and move it up and down. 

The pulley should move less than 1mm, and you can also try starting the alternator by hand and feel if the bearings are dry or worn. Detecting excessive bearing play and listening to dry bearings usually requires some experience and is difficult to describe in words.